This application claims priority to prior application JP 2002-46248, the disclosure of which is incorporated herein by reference.
The present invention relates to a solid electrolytic capacitor and a manufacturing method for the same and, more particularly, to a technology effective for achieving a thinner design and a reduced equivalent series resistance (ESR) of a surface-mounted solid electrolytic capacitor.
In recent years, there has been accelerating trend toward compactness, lighter weight, and portability of electronic equipment. With this trend, there has been increasing demand for smaller and thinner electronic components. Portable devices, in particular, are required to be small and thin at the same time. There are severe restrictions on the thickness of the electronic components used with portable devices, meaning that the demand for making thinner electronic components is high.
Under the aforesaid circumstance, not only semiconductor components but passive components, such as electrolytic capacitors, in particular, used for decoupling or the like in a power circuit are required to be smaller and thinner. Hitherto, most of this type of capacitors are made by coating small capacitor elements with a molding resin to form them into small surface-mounted capacitors, which are so-called xe2x80x9cchip capacitors,xe2x80x9d as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 62-005630, Japanese Unexamined Patent Application Publication No. 58-157125, or Japanese Unexamined Patent Application Publication No. 4-123416. Frequently, plate-like or foil-like materials are used for the capacitor elements in order to achieve a thinner design. Conventional capacitors are structurally characterized by the coating with a molding resin.
FIG. 1 shows the section of a surface-mounted solid electrolytic capacitor according to a conventional embodiment. Referring to FIG. 1, a capacitor 1 is constructed of a plate-like anode member 2, a cathode conductor layer 3 covering the majority of the anode member 2, an anode terminal 4 secured to a portion of the anode member 2 that is not covered by the cathode conductor layer 3, a cathode terminal 5 secured to the cathode conductor layer 3, and a coating resin layer 9.
The anode member 2 uses, as its base material, a plate or foil (hereinafter, xe2x80x9cplatexe2x80x9d will include xe2x80x9cfoilxe2x80x9d unless otherwise specified) of aluminum or tantalum, or an oxide film forming valve metal, such as niobium. The area of the surface of the base material metal plate is expanded by, for example, etching. A metal oxide film (not shown) of the base material metal is deposited on the expanded surface by, for example, anode oxidization. The metal oxide film provides the dielectric of the capacitor.
The cathode conductor layer 3 is constructed of, for example, a solid electrolyte layer, a graphite layer, and a silver paste layer (none of them being shown), which are deposited in this order on the metal oxide film on the surface of the anode member. The solid electrolyte layer uses a conductive polymer, such as polypyrrole, polythiophene, or polyaniline, or a semiconductor material, such as manganese dioxide.
The anode terminal 4 is conductively fixedly connected to the anode member 2 by, for example, laser welding or resistance welding. Meanwhile, the cathode terminal 5 is bonded to the silver paste layer, which is the topmost layer of the cathode conductor layer 3. These anode terminal 4 and the cathode terminal 5 are used for external electrical connection.
The coating resin layer 9 is formed by transfer molding in which a thermosetting resin, such as epoxy resin, is used as the material. The coating resin layer 9 covers the anode member 2, the cathode conductor layer 3, the joint portion between the anode member 2 and the anode terminal 4, and the joint portion between the cathode conductor layer 3 and the cathode terminal 5. The coating resin layer 9 blocks the entry of oxygen and/or moisture from outside.
The anode terminal 4 and the cathode terminal 5 are bent along the side surfaces of the coating resin layer 9, and further bent inward on the bottom surface of the coating resin layer 9, i.e., the mounting surface of the capacitor. The portions of the anode and cathode terminals 4 and 5, respectively, which face each other, are the portions for external connection.
The capacitor is mounted on a mounting wiring board 6 by soldering the portions of the anode and cathode terminals for external connection to lands 7a, and 7b, respectively, of the wiring pattern formed on the wiring board.
As described above, this type of capacitors has conventionally been securing reliability by coating them with a molding resin so as to protect them from external oxygen and/or moisture.
However, when the resin transfer molding is carried out, it is necessary to secure a gap of a certain size or more between the inner wall of a metal mold and a capacitor element in the metal mold in order to allow molten resin to flow in the metal mold. This means that the coating resin layer 9 will always have a thickness of a certain value or more. If a gap of a sufficient thickness for the flow of molten resin is not provided between the capacitor element and the metal mold, then the gap will have a portion not filled with the resin. As a result, the finished capacitor will have a defective portion uncovered by the coating resin layer 9, leading to failure. Such a defective portion will not be capable of blocking oxygen and/or moisture from outside, resulting in deteriorated reliability of the capacitor. Hence, there is limitation in reducing the thickness of a capacitor.
As shown in FIG. 1, the anode terminal 4 and the cathode terminal 5 are temporarily drawn out of the coating resin layer 9, the distal ends of the drawn-out terminals provide external connection. Therefore, the resistances of the anode terminal and the cathode terminal inevitably increase accordingly with a resultant increase in the ESR of the capacitor.
Accordingly, it is an object of the present invention to provide an electrolytic capacitor that prevents the entry of oxygen and/or moisture from outside, exhibits higher reliability, and has a reduced thickness, as compared with a conventional capacitor coated with a molding resin.
It is another object of the present invention to provide an electrolytic capacitor having lower ESR.
To these ends, according to the present invention, there is provided a solid electrolytic capacitor including an anode terminal and a cathode terminal for external electrical connection that are formed on one surface of a plate-like or foil-like anode member; a cathode conductor layer formed such that it covers the area of the one surface of the anode member except for the portion where the anode terminal is secured; a first metal plate or metal foil functioning as the cathode terminal that is closely joined to the one surface of the cathode conductor layer so as to cover the one surface of the cathode conductor layer; and a second metal plate or metal foil closely joined to the other surface of the cathode conductor layer so as to cover the other main surface of the cathode conductor layer; wherein the ventilation between the anode member and the outside is blocked by the first metal plate or metal foil or the second metal plate or metal foil.
According to another aspect of the present invention, there is provided a manufacturing method for a solid electrolytic capacitor having an anode terminal and a cathode terminal for external electrical connection that are formed on one surface of a plate-like or foil-like anode member, the manufacturing method including a step of forming the anode member by increasing the area of a plate-like or foil-like valve metal and depositing a layer of oxide of a base material valve metal on the surface of the increased area; a step of forming a cathode conductor layer formed such that it covers the area of the one surface of the anode member except for the portion where the anode terminal is secured; a step of electroconductively securing the anode terminal to the portion of the anode member that is not covered by the cathode conductor layer; a step of bonding, to one surface of the cathode conductor layer, a first metal plate or metal foil that has an area larger than the area of the cathode conductor layer on that surface side and functions as the cathode terminal; and a step of bonding, to the other surface of the cathode conductor layer, a second metal plate or metal foil having an area larger than the area of the cathode conductor layer on that surface side.